The present disclosure relates to a combustor, and more particularly to a combustor with a cooling air mixture that reduces peaks in exit gas temperatures and reduces emissions simultaneously.
As gas turbine engine design requirements increase for improved thrust specific fuel consumption (TSFC), compressor discharge conditions of pressure and temperature along with combustor exit temperatures (CET) may increase. As a result, current combustor configuration emissions, such as NOx, CO, unburned HC, and smoke, may increase. Emissions such as smoke are derived from fuel rich regions with high temperature gradients as unburned carbon. CO is an intermediate product of HC combustion, formed in rich flames with insufficient oxygen or in lean flames due to excessive quenching. NOx emissions can be classified in three categories: (1) thermal NOx associated with increases in flame temperature, proportional to the residence time in the combustor; (2) fuel NOx associated with conversion of fuel bound nitrogen in the fuel; and (3) prompt NOx associated with interactions of transient chemical species (typically HC) in the flame front with surrounding nitrogen. These emissions are related to flame temperature profiles, and to flame stability. As such, reduction of residence time after one or more stages of combustion may minimize thermal NOx, and reduce exit gas temperature distributions.